Extract user id from headers for user-specific queries instead of passing manually

Description of changes

Uses the new functionality where user data is transmitted via the headers to get the current user id for queries/mutations depending on it.

Please describe the test strategy you followed.

Writing automated tests will be done in the next issue.

[x] My code follows the coding guidelines of this project
[x] I have performed a self-review of my code
[x] I have commented my code, particularly in hard-to-understand areas
[x] My code fulfills all acceptance criteria
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] New and existing unit tests pass locally with my changes
[ ] The test coverage (line and branch) is reasonably high, especially on Service classes and other classes with complex logic
[ ] I have made corresponding changes to the documentation
[ ] I have added explanation of architectural decision and rationales to wiki/adr
[ ] I have updated the changes in the ticket description

[ ] The code works and does not throw errors
[ ] The code is easy to understand and there are no confusing parts
[ ] The code follows the coding guidelines of this project
[ ] The code change accomplishes what it is supposed to do
[ ] I cannot think of any use case in which the code does not behave as intended
[ ] The added and existing tests reasonably cover the code change
[ ] I cannot think of any test cases, input or edge cases that should be tested in addition
[ ] Description of the change is included in the documentation